ARM: dts: Add NextThing GR8 dtsi

The GR8 is an SoC made by Nextthing loosely based on the sun5i family.

Since it's not clear yet what we can factor out and merge with the A10s and
A13 support, let's keep it out of the sun5i.dtsi include tree. We will
figure out what can be shared when things settle down.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>